The present invention relates to an electric double layer capacitor of high electrostatic capacitance which utilizes the principles of an electric double layer.
Recently, high-capacitance capacitors based on the electric double layer principles have been developed as backup power supplies for memories in electronic systems, and are widely used with microcomputers and IC memories.
One type of electric double layer capacitor is disclosed in U.S. Pat. No. 3,536,963, for example. FIG. 6 of the accompanying drawings shows the structure of the disclosed electric double layer capacitor. The electric double layer capacitor comprises a single basic cell composed of a pair of current collectors 30 of an electron conductor which serves as a pair of collector electrodes, a pair of carbon electrodes 20 made of activated carbon particles, a nonconductive gasket 10, and a separator 40 positioned between the carbon electrodes 20 for preventing electrons from moving between the electrodes 20. The carbon electrodes 20 are made as paste electrodes from a concentrated slurry which is a mixture of powdery or particulate activated carbon and an electrolyte. The electrolyte has three functions to perform. It serves as a promoter of ion conductivity, an ion source, and a binder for the carbon particles.
It is important that the internal resistance of an electric double layer capacitor be low. The internal resistance of an electric double layer capacitor is greatly affected by the contact resistance of active carbon of the polarized electrodes and the contact resistance between the collector electrodes and the polarized electrodes.
Therefore, in order to reduce the internal resistance of the polarized electrodes and the contact resistance between the collector and polarized electrodes, each basic cell should be kept under vertical pressure to bring the particles of the paste activated carbon into good electric contact with each other. Conventional electric double layer capacitors require each cell to be kept under a pressure of about 100 kg/cm.sup.2 though it depends on the size of the electrodes, the size of the particles of the carbon material, or the kind of the electrolyte used. In prior electric double layer capacitors, the cells are kept under pressure by deforming the outer cases of the capacitors or bonding the current collectors strongly to gaskets. If an electric double layer capacitor is to be used as a large-capacitance capacitor, e.g., a power supply for energizing a motor, then it is necessary to increase the cross-sectional areas of the electrodes of the basic cell. Therefore, the pressure to be applied to the basic cell has to be increased. Increasing the pressure, however, causes some practical problems such as the selection of means for applying the pressure and the need for high rigidity for the outer cell which houses the basic cell.
To solve the above problems, the applicant has proposed an electric double layer capacitor having polarized electrodes each in the form of a porous sintered body and current collectors each made of an electrically conductive material which are composed of electrically conductive fine particles mixed in a base, the current collectors being attached to one surface of the polarized electrodes and entering pores of the polarized electrodes (see Japanese patent application No. 1(1989)-232243. Since the polarized electrodes are constructed as thin plates of porous sintered bodies, the electric double layer capacitor is of a small internal resistance. If the electrodes are increased in size for a higher electrostatic capacitance, then the mechanical strength thereof is reduced because the porous sintered bodies are low in density, and hence the electrodes are liable to be broken when subjected to external bending or torsional loads. Production of large-size sintered electrodes requires expensive dies, and hence is problematic with respect to cost.